1. Field of the Invention
The present invention relates to an isolation method for a semiconductor device, and more particularly, to a trench isolation method in which the concentration of an electric field can be prevented by forming an upper corner of a trench to be rounded and isolating the semiconductor devices by forming a trench in a semiconductor substrate.
2. Description of the Background Art
A local oxidation on silicon (LOCOS) is chiefly used as a conventional method for isolating semiconductor devices. However, the trench isolation method is receiving much attention, since the trench isolation structure does not encroach a channel width, the size of a device can be advantageously reduced. Also, such trench isolation structure can prevent latchup effects, and thus the characteristics of the semiconductor memory device can be improved, and in addition, the trench isolation method allows a uniform structure to be formed.
As shown in FIG. 1, according to the conventional trench isolation structure, a trench 5 is formed in a substrate 1, an upper portion of the trench 5 is not formed to be rounded, and a TEOS oxide 6 formed by a chemical vapor deposition (CVD) is filled in the trench 5. The TEOS oxide 6 has an upper surface formed in a double-hump structure.
In the above-described isolation method of a trench structure, first, an oxide film 2 and a nitride film 3 are sequentially formed on the semiconductor substrate 1, as shown in FIG. 2A, and a portion of the surface of the semiconductor substrate 1 is exposed by performing a partial etching on the nitride film 3 and the oxide film, as shown in FIG. 2B. Then, as shown in FIG. 2C, the trench 5 is formed by etching the exposed semiconductor substrate 1 to a predetermined depth, and as shown in FIG. 2D, the trench 5 is filled with the oxide 6 about up to a level of the nitride film 3. Finally, the nitride film 3 and the oxide film 2 are removed.
However, in the conventional trench isolation structure, since the upper corner of the trench 5 is not formed to be rounded, an electric field is concentrated at the corner which deteriorates the characteristic of the device. Also, since a double hump occurs in that the portion where the trench-filling oxide 6 is in contact with the upper corner is sunken, when the patterning is carried out after polysilicon (not illustrated) for a gate electrode is deposited in the trench 5, the sunken portion of the double hump structure formed in the trench 5 is filled with polysilicon. Thereby, a bridge is formed instead of a gate electrode (not illustrated), and as a result, the device is not operational.
Accordingly, to solve the above-described problem, recently, a method for rounding the corners of the trench has been considered, and one example of a trench isolation method was disclosed in 1996 Symposium on VLSI Technology Digest of Technical Papers p.156-157.
That is, a twin well is formed by a general diffusion on a p-type silicon substrate which has 5-15 .OMEGA./cm epitaxial resistance and a thickness of 3.5-6 .mu.m, and then a pad oxide film is formed to have a thickness of 100-150 .ANG. on the semiconductor substrate. Then, a nitride film is formed to have a thickness of 1500-2000 .ANG. on the pad oxide film, and an active region and a non-active region are defined by a general photolithography process on the nitride film. Then a shallow trench etching is carried out as deep as 0.5 .mu.m on the non-active region. Next, the semiconductor substrate is thermally oxidized with 5% HCl at a temperature of 900.degree. C. to form a thermal oxide film having a thickness of 200 .ANG.. Then 9000 .ANG. of a high density plasma chemical vapor deposition (HDP CVD) oxide film is deposited on the thermal oxide film in the trench, and a chemical mechanical polishing (CMP) is carried out on the entire surface of the semiconductor substrate to remove the nitride film. Then, the pad oxide film is eliminated, and a gate oxide film is formed. Using the gate oxide as a mask, an impurity implantation is performed to form a source and a drain.
However, according to the this disclosed shallow trench isolation method, when the corner of the trench is rounded, the oxidation process must be carried out under a HCl atmosphere, but the HCl gas is a material hazardous to the environment.